This invention relates to an electronic musical instrument, and more particularly an electronic musical instrument of the waveshape memory read out type.
As is well known in the art, an electronic musical instrument of the waveshape memory read out type is constructed to repeatedly read out sampling point amplitude values in one period of a musical tone waveshape stored in a waveshape memory device over a period corresponding to the tone pitch of a depressed key and then convert the read out sampling point amplitude values to corresponding analogue signals so as to produce a musical tone having a frequency corresponding to the tone pitch of the depressed key. Since the waveshape stored in the waveshape memory device corresponds to a specific tone color, it is impossible to change the tone color of the generated musical tone to another tones.
To solve this problem, one may prepare a plurality of waveshape memory devices respectively storing musical tone waveshapes corresponding to different tone colors and to switch these waveshape memory devices for producing musical tones of different tone colors. However, provision of a plurality of waveshape memory devices not only increases the cost but also increases the size of the instrument. In certain cases, some of the waveshape memory devices may not be used to that the efficiency of the memory devices is poor.
For this reason, it has been proposed to use a random access memory device (RAM) as the waveshape memory device, to write into the RAM a musical tone waveshape of a desired tone color and then sequentially read out the written content for generating a musical tone, as disclosed, for example, in U.S. Pat. No. 4,142,432, issued on Mar. 6, 1979.
In an electronic musical instrument of the type described above utilizing a RAM, that is of the type wherein the musical tone waveshapes are renewed or rewritten, if the content of the RAM is renewed quickly to a musical tone waveshape to be generated next time, noise in the form of clicks generates thus giving unpleasant feeling to the listeners. To solve this problem, in an electronic musical instrument disclosed in said patent, an interpolation is made for the purpose of removing click noise which generates at the time of renewal of the musical tone waveshape from old one to new one. More particularly, according to this system, at the time of renewal, a value which decreases gradually is multiplied to an old musical tone waveshape now being generated while a gradually increasing value is multiplied to a new musical tone waveshape to be generated next and the sum of the products is produced as a musical tone waveshape signal thus ensuring smooth renewal of the musical tone waveshape. This renewal system, however, not only requires a long arithmetical operation time, but also requires circuits for generating various timing pulses utilized to execute the arithmetic (multiplying) operation thereby complicating the circuit construction. In addition, in the electronic musical instrument described above, as the renewal rate of the musical tone waveshape is fixed, tone color changes rapidly at the time of renewal thus giving unnatural feeling to the listeners in some cases.